Starfish - FT
OCHRE STARFISH (Pisaster ochraceus) Starfish or sea stars are star-shaped echinoderms belonging to the class Asteroidea. They are marine invertebrates and usually have a central disc and five arms; the disk at the center of their legs holds their vital organs. They are found from the intertidal zone (seashore) down to abyssal depths, 6,000 m below the sea surface. Ochre Starfish, in particular, are found in the intertidal zone along the Pacific North American coast. This can be problematic for them, as when tidewater recedes they can be left exposed to the hot sun and atmospheric air until high tide. Starfish have a definite limit on core temperature—if it reaches 35oC they die, as their vital organs can no longer function. Therefore, it is important for them to maintain a stable core temperature, which is an example of homeostasis, despite of changes in environmental temperatures. Since they are ectotherms, they have developed a range of mechanisms to “exploit” their surroundings in order to regulate their temperature (thermoregulation). Anatomical * Water vascular system- Cold water from the sea enters the core of the star fish where it passes from a series of tubes that extend from the stomach to the arms to transport nutrients. This is what starfish have instead of a circulatory system. * Flat and spread out body- Large surface area for absorption of heat and sunlight. * Large surface area-to-volume ratio- effective removal of heat. * Light, bright colours (starfish that live on the seashore)- They reduce heat absorption, as some of the sunlight is reflected away from the body. This is beneficial during low tide when all of their body is continuously exposed to the sun. * Tube feet on the underside of its body- They allow the starfish to move short distances and position themselves in a desired position relative to the sun or tide level. They, also, allow them to flatten their body to increase or decrease surface area for heat absorption or heat loss. Physiological/ Behavioural * The starfish uses its arms as heat sinks- The temperature in the legs of the starfish is higher than that of their core by anywhere from 3˚C to 5˚C. The animals use their legs as heat sinks; they actively divert heat from the core into their arms, by moving fluid within themselves, similarly to how mammals transport blood around their circulatory system. They can then release the heat into the water relatively efficiently because of their large surface area and small internal volume. In extreme cases, the diversion of heat to arms can thermally damage the arms beyond repair, when they may shrivel up and fall off—they can later be regenerated. * The starfish pumps itself up with cold seawater to lower its body temperature when exposed to the sun at low tide, when it can suffer heat stress- As mentioned above, the ochre starfish is found in the intertidal zone along the Pacific North American coast. During low tide it is exposed to the air and cannot move until it is submerged again at high tide. During the days after exposure to high temperature at low tide, the starfish sucks up water through its vascular system, which increases its body mass. The increase in body mass allows the starfish to be buffered against environmental temperature changes in the subsequent low tide; because its body mass is higher, body temperature increases more slowly. By varying the amount of water they take up in their vascular system and body cavity they can adjust to the varying levels of water availability and environmental temperature, which allows them to have a more sophisticated thermoregulatory mechanism than many ectotherms.